26 ABB Review 4/2000

pproximately 35 years ago, in the

mid 1960s, two new breaker

technologies, one using SF6 gas and the

other vacuum as its arc quenching

medium, were introduced to the market.

Research and development work on

both technologies has continued

unabated since then, and today it can

be said that, together, they have all

but replaced the older types of switch-

gear. There is, however, not always

agreement on which criteria should

be used when choosing one of these

two dominant technologies. Instead of

an objective selection based on real-

world characteristics, the choice is very

much driven by the circuit-breaker

manufacturer.

Guenter Leonhardt, Mauro Marchi, Giandomenico Rivetti

More than three decades of experience in developing SF6 and vacuum circuit-breakers, accompanied

by increasingly close cooperation between the participating research centers, gives ABB a strong

advantage when it comes to deciding which technology is best for a given application. The

pioneering role played by the company uniquely qualifies it to pursue R&D on both fronts with the

goal of pushing performance levels to the limit. The sum of this research work, coupled with

unrivalled knowledge of the marketplace, puts ABB in a position to offer unbiased advice and

assistance to customers searching for the switchgear that best suits their needs.

A

Choosing the right MVcircuit-breaker

SF6orvacuum?Switchgear

ABB Review 4/2000 27

SF6 and vacuum switchgear enjoy

varying market success in the different

parts of the world ; whereas Europe

and most of the Middle East countries

tend to favor SF6, China, Japan and the

USA definitely prefer vacuum. In other

regions, the two technologies are equally

popular. Bulk-oil and minimum-oil

technologies are still used in China,

Eastern Europe, India and Latin America,

but trends clearly indicate that these

technologies will disappear very soon, to

be replaced by SF6 and vacuum.

As shows, ABB concentrates today

almost entirely on the two dominant

technologies, and is equally present in the

market with both SF6 and vacuum.

Experience with more than

300,000 MV circuit-breakers of both

designs installed worldwide, backed up

by over 30 years of intensive involvement

in research [1], has convinced ABB that

the two technologies are entirely

complementary, though in some cases

their different designs can be seen as

alternatives. Based on this conviction that

SF6 and vacuum have equally important

roles to play, the company has continued

to force the development of both, and

hence, as the world’s largest manufacturer

of MV circuit-breakers, occupies the

unique position of being able to provide

unprejudiced advice and assistance in the

selection of switchgear for any special

application.

The decision by ABB to pursue both

technologies with equal emphasis has

produced several important benefits. First

and foremost, a profound knowledge of

the behavior of the two technologies has

led to better service for customers. At the

same time, keen competition between the

company’s research laboratories has led

to top team performances, the exchange

of information between them having

extracted the maximum synergy from

parallel work. Finally, it was recognized at

an early stage that it would be of great

advantage to both user and manufacturer

if the circuit-breakers were constructed so

as to be completely interchangeable, as

shown by .

Proceeding in this way, all new

developments are equally advantageous

to both technologies. The most important

developments to come out of this

3

2

1

USA

Latin A

meric

a

Weste

rn E

urope (C

APIEL)

Central &

Easte

rn E

urope

Mid

dle East &

North

Afri

ca

China

India (I

EEMA)

Asia

[%]

Others

SF6

Vacuum

100

80

60

40

20

0

100

80

60

40

20

0

[%]

(SF6+vacuum) SF6

Others Vacuum

199919981997199619951994199319921991

1 Worldwide market for MV circuit-breakers by region (1998)

2 MV circuit-breakers manufactured by ABB

28 ABB Review 4/2000

approach are the use of magnetic

actuators as the operating mechanism and

the integration of sensors in the switchgear

panels. With total interchangeability,

users are faced with easier choices and

structural factors are no longer a major

consideration in their decisions.

Arc-interrupting characteristics

SF6 circuit-breakers

Sulfur hexafluoride (SF6) is an artificial

inert gas with excellent insulating

properties and exceptional thermal and

chemical stability. These characteristics

of the gas have led to its widespread use

in both HV and MV switchgear, both of

which exhibit very high performance

and reliability as a result.

The specific advantages of SF6 gas in

electrical engineering applications have

been widely recognized since the early

1930s, but it was only in the late 1950s

that the first high-voltage SF6 insulated

circuit-breakers were developed and

installed. SF6 medium-voltage circuit-

breakers followed some years later.

The first generation of MV SF6 circuit-

breakers employed a dual-pressure gas

system. Second-generation designs

included the pressure differential

necessary to create the gas flow, this

being provided by a mechanically driven

piston which compressed a small volume

of gas. The piston was integrated in the

moving contact assembly. Such ‘puffer-

type’ circuit-breakers required a relatively

powerful mechanism [2]. The third

generation of designs produced the gas

flow by utilizing the energy contained in

the arc. This ‘self-blast’ circuit-breaker

design resulted in significantly less energy

being required for its operation.

The more than 30 years of ABB

experience and research associated with

the puffer and self-blast circuit-breakers

have now culminated in a new and very

efficient design. This so-called ‘auto-

puffer’ combines the advantages of both

previous designs. The auto-puffer circuit-

breaker operates as a pure puffer device

when interrupting currents up to 30% of

the maximum rated breaking capacity and

as a self-blast interrupter at higher levels.

The auto-puffer requires only a minimum

amount of energy from the operating

mechanism, but offers the high

3 VM1 vacuum (left) and HM1 SF6 medium-voltage circuit-breakers with magnetic actuator

Switchgear

performance levels of the self-blast type.

Reduced arc energy dissipation at both

low and high (short-circuit) currents

ensures a longer electrical life than either

of the former designs. This performance

is obtained without jeopardizing the total

absence of chopping currents, which is a

key characteristic of the self-blast

technique. The design of the mechanism

has been optimized to generate only

enough pressure to ensure the safe

interruption of currents in the range in

which the puffer technique is operative.

Consequently, small inductive currents are

effectively interrupted with overvoltage

factors lower than 2.5 pu.

Vacuum circuit-breakers

As early as the beginning of the last

century, the interruption of current in a

vacuum was recognized as an ‘ideal’

switching technique. However, several

practical difficulties led to it being

ignored for almost three decades. One of

the fundamental problems was the

manufacture of a suitable insulating

enclosure, which had to be hermetically

sealed for life. This problem existed

through a number of decades until, in

the early 1960s, a solution using glass

enclosures was developed. Curiously, the

fundamental technology of blown-glass

containers had been commonly available

for centuries. A further step forward came

with the development of alumina (Al2O3)

ceramics, a material which possesses a

much higher resistance to cyclic

temperature stresses.

Finding a suitable material and form

for the circuit-breaker contacts was also a

considerable problem. The contacts had

to exhibit a high resistance to arc erosion

during both opening and closing

operations, and any erosion had to be

diffuse and even over the whole contact

surface. The contact material had to have

a low propensity to weld during closing

as well as when closed. Low current

chopping characteristics when

interrupting small currents was also

important, as was an adequate gettering

effect. The search for a suitable material

showed that chromium possessed most

of the required properties. Further

research showed composite material of

copper/chromium to be the most suitable

and best able to satisfy the basic

requirements. Cu/Cr with a chromium

content of between 20% and 60% is now

the standard material for contacts, and is

used by all manufacturers of vacuum

circuit-breakers.

The mechanism of charge carrier

formation gives a vacuum circuit-breaker

1

2

3

4 Cutaway view of the magnetic actuator

1 Coils

2 Permanent magnets

3 Plunger

ABB Review 4/2000 29

30 ABB Review 4/2000

the inherent ability to extinguish current

arcs of small to medium values

automatically when the current passes

through zero. A satisfactory interruption

of short-circuit currents, however,

requires additional design measures.

The initial designs used a specially

shaped electrode to produce a radial

magnetic field in the arc contact area.

This magnetic field, reacting with the arc

current, forced the arc root to move

continually around the contact surface,

thus preventing local overheating and

uneven wear.

A further design improvement, aimed

particularly at increasing the current

interrupting capacity to extremely high

short-circuit currents, was the

development of the ‘axial’ magnetic field.

Again a specially designed electrode is

employed to generate an axial magnetic

field, which distributes the arc root

homogeneously over the whole of the

contact area.

Common trends in SF6

and vacuum circuit-breaker

development

ABB SF6 and vacuum circuit-breakers

have been used for many years in

medium-voltage switchgear and service

experience has shown them to be

reliable, almost maintenance-free and

safe under operating conditions.

Innovations in both technologies have

continually improved their efficiency,

reduced their overall dimensions

and, most importantly, reduced the

amount of energy required to operate

them.

This reduction in operating energy

has led to the development of an entirely

new design of operating mechanism,

the permanent magnet actuator.

Magnetic actuator

The operating mechanism of a circuit-

breaker has the apparently ‘simple’

function of moving the contacts from the

closed to the open position or vice versa

and, when the required position is

reached, of ensuring that the contacts

remain in that position until a definite

command to again change position is

given. The operating mechanism is thus a

typical bistable actuator. This function has

been performed with a high degree of

reliability and surety for many years by

mechanical spring and latch mechanisms.

However, the opportunities now offered

by developments in power electronics

have led to the search for a more flexible

and more readily controllable operating

device. Of course, an essential prerequi-

site of any new system was that it had

to guarantee at least as good or better

performance, in terms of reliability, safety

and durability, than the traditional spring-

based mechanism.

An appropriate solution has been

found in the ‘magnetic actuator’.

A specially designed system combining

electromagnets with permanent magnets

provides the operating energy for the

movement of the contacts as well as the

essential bistable characteristic. The

vacuum and the SF6 interrupters are held

in either their open or closed positions by

the force of a permanent magnet and this

without the need for any external energy.

The change of status of the moving

contacts is brought about by a change

in direction of the magnetic field

resulting from the energizing of the

electromagnets, which are the control

elements of the actuator. Modulation of

the current supply to the electromagnets

allows the energy developed by the

system to be adjusted to the requirements

of different types and ratings of circuit-

breakers.

The resulting operating mechanism is

considerably simpler in construction than

the conventional mechanical system. The

drastic reduction in the number of parts

inherently reduces the susceptibility to

failure, and the level of maintenance

required by this operating mechanism is

reduced to the very minimum. shows

the construction of such an actuator with

its fixed laminated iron core, permanent

magnets, steel armature and closing and

opening coils. All auxiliary functions,

such as interlocking, signaling, tripping,

closing, etc, are provided electronically;

self-diagnostic facilities are also included.

An electrolytic capacitor provides the

surge power required for the opening

and closing coils.

Basic construction of the

switching devices

The new vacuum and SF6 magnetically

actuated circuit-breakers are fully

interchangeable with each other as well

as with previous designs. This

interchangeability is of considerable

importance to plant operators as it allows

existing switchgear to be re-equipped at

minimum cost.

4

Switchgear

ABB Review 4/2000 31

and show clearly the very

small number of components used, a fact

that significantly reduces the potential for

failure.

Simplicity is also a feature of the em-

bedded vacuum pole and the SF6 inter-

rupter using the auto-puffer technique,

specially adapted for medium-voltage

applications .

Thanks to the embedding technology

there is no need for special support struc-

tures for the interrupter or its terminals.

Rapid switching

An important property of the magnetic

actuator that has already been mentioned

is the versatility of its control functions.

Exploitation of this flexibility opens the

door to new solutions to key problems

in electrical distribution, problems which

until today have been solvable, if at all,

only at great expense. One of these

issues is the rapid transfer-switching

between energy sources in the event of a

fault in one system. This problem has

been dramatically emphasized in the last

few years by an exponential increase in

power-quality sensitive loads, mostly due

to the use of electronic equipment. The

present solution, based on power

electronics devices, is very efficient from

the technical point of view but also very

expensive. Introduction of the magnetic

actuator has made it possible to

accelerate the operation of an MV circuit-

breaker to the absolute minimum, that is

to the pure arc extinction time. Using

magnetically actuated medium-voltage

circuit-breakers and appropriate basic

electronics, it has been possible to

reduce power source transfer-switching

times to less than 40 ms. This elapsed

time is so short that it solves most of the

problems of sensitive loads and at a

cost which is very competitive compared

with the power electronics based

solutions [5].

Synchronous circuit-breaker

The availability of these new circuit-

breakers with their magnetic actuator

mechanisms has another important

advantage: they provide the basis for

synchronous switching. This switching

technique involves the circuit-breaker

poles being independently operated,

with each pole opened or closed at the

best point in time relative to the current

and/or voltage conditions prevailing in

the relevant phase. Synchronous

switching minimizes the electrical and

mechanical stresses which arise on both

the supply and load sides of the circuit

being switched and in the circuit-breaker

itself when the current is interrupted.

With synchronous switching, the amount

of energy which has to be dissipated in

the interruption chamber is minimized

and any overvoltage resulting from the

switching operation is greatly reduced.

All of these advantages result from the

precise control of the circuit-breaker

operation made possible by the magnetic

actuator. The control accuracy is so great

that it is possible to synchronize the

completion of the moving contact travel

with the current zero-crossing in each

phase. Furthermore, synchronous

switching minimizes, theoretically even

reduces to zero, the inrush current peaks

and overvoltages that occur during the

energization of inductive or capacitive

loads. Given the type of load, these

results are obtained by controlling the

closing of the contacts to correspond

with either the current or voltage

maximum. The closing and opening

operations described are performed with

a maximum tolerance of ± 0.5 ms and

± l ms, respectively. These figures are a

true measure of the value of the

technological breakthrough achieved by

the combination of digital electronics

with the magnetic actuator.

6

54

5 View of the VM1 vacuum (right) and HM1 SF6 medium-voltage circuit-breakers

showing the small number of components

These developments will result in

improved reliability for the entire

electrical system, greater safety for the

personnel, and cost reductions due to the

minimization of electrical stress and wear

on the electrical equipment [6].

Integration with sensors

and electronics

The hardware and software presently

available for use with the magnetically

actuated circuit-breakers allow a further

step forward in the direction of complete

functional integration. With the appropriate

software and the necessary current and

voltage sensors, the direct integration of

the protection functions in the circuit-

breaker control system is now possible.

This makes the circuit-breaker a fully

automated device for protection and

switching functions and achieves the

goal of maximum reliability – the result

of minimization of the component inter-

faces. This total integration of the core

functions in switchgear has already been

shown to be the correct path to follow in

medium-voltage secondary distribution

applications, just as it is already the

current state of the art in the field of low

voltage equipment.

Technical performance

Electrical and mechanical

endurance

Both SF6 and vacuum circuit-breakers

can be considered maintenance-free.

High-quality SF6 circuit-breakers as well

as high-quality vacuum circuit-breakers

fulfil the requirements for class B circuit-

breakers as given in the IEC 60056

standard [3]. This states that:

‘A circuit breaker class B (in the IEC

draft document, in the future it will be

E2) is a circuit breaker designed so as not

to require maintenance of the interrupting

parts during the expected operating life of

the circuit-breaker, and only minimal

maintenance of its other parts.’ Based on

service experience, the IEC standard

establishes the number of operations that

a circuit-breaker shall be capable of

performing under the severe service

conditions associated with an overhead

line connected network and including

auto-reclosing duty.

The standard prescribes two

alternative test cycles for the verification

of electrical endurance performance of a

circuit-breaker. The test cycle in

accordance with List 1 is the preferred

one; the test cycle of List 2 may be

applied as a valid alternative for circuit-

breakers for use in solidly grounded

systems. The severity level of these two

test cycles is regarded as identical.

Reliability of dielectric

media

Modern SF6 and vacuum circuit-breakers

are sealed for life; diagnostic systems for

the purpose of measuring the gas

32 ABB Review 4/2000

6 Section through the embedded vacuum pole (left) and the auto-puffer SF6 pole

Switchgear

ABB Review 4/2000 33

pressure or the vacuum level are

therefore unnecessary.

Switching overvoltages

Any switching overvoltages generated by

circuit-breakers using either technology

are contained within such limits as not to

present any danger to connected

equipment or installations.

Due to their inherently soft interruption

characteristics SF6 circuit-breakers offer

this level of performance without the

need of any additional devices.

Vacuum circuit-breakers using modern

contact materials also exhibit low chopping

currents; however, in exceptional cases,

and depending on the characteristics of

the individual installation, a detailed study

of the system parameters may be

necessary in order to determine if specific

voltage limiting devices are required.

Environmental impact

The operation of either circuit-breaker

type presents no health hazard to

personnel. In the unlikely event of a

major malfunction, overpressure valves

built into the SF6 circuit-breakers would

respond, while vacuum circuit-breakers

would be subject to no more than

implosion phenomena. Experience has

also shown that any emission products

from either type of circuit-breaker do not

constitute a toxic hazard. The component

materials of both types of apparatus can

be readily recycled at the end of their

service lives. The Kyoto Protocol to the

United Nations Framework Convention

on Climate Change (10th December 1997)

has established that emissions of six gases

considered to be a likely cause of global

warming, SF6 among them, need to be

reduced. It was therefore necessary to

analyze the greenhouse gas (ie, SF6 and

CO2) emissions occurring as a

consequence of the manufacturing

process and the power losses in service.

The Life Cycle Assessment (LCA) that was

subsequently carried out for vacuum and

SF6 circuit-breakers leads to the following

conclusions, which are substantially the

same for both types of equipment.

The impact of the manufacturing and

the service phases are to be considered

separately. Consideration of the SF6

circuit-breaker shows that the

environmental impact during the entire

manufacturing phase is more than

100 times greater than the environmental

impact of the unit throughout a 30-year

total life cycle due to the fact that

medium-voltage SF6 breakers are sealed

for life [4]. The production of the copper

and insulating components of the circuit-

breaker is the predominant contributor to

the environmental impact throughout the

manufacturing phase.

As regards the environmental impact

during service, based on an assumed

30-year service life and an average load

current of 20% of the rated current it can

be calculated that the service phase has

an environmental heating effect of more

than 7 to 8 times that caused during the

manufacturing phase. This is due to the

resistance losses in the circuit-breaker.

The analysis shows that the environmental

impact of the SF6 gas itself, relative to the

impact of the complete apparatus over its

complete life cycle, is only about 0.1% of

the total. When considering vacuum

circuit-breakers, it is evident that because

of the quantity of copper and the number

of insulating components, as well as the

main circuit resistance, the results are

very close to those for the SF6 circuit-

breaker.

Considering the global warming effect

alone, it can be concluded that the impact

is determined essentially by the main

circuit power losses. However, these

losses are in turn fully negligible when

compared with those caused by the

cables, connections and all the other

apparatus which make up the electrical

distribution system.

Specific switching applications

Overhead lines and cables

When applied to the onerous duty of

switching and protecting overhead line

distribution networks, in which the fault

currents are distributed over the whole

current range, both technologies provide

adequate margins over and above the

maximum required by the relevant

standards and in normal service practice.

Transformers

Modern vacuum circuit-breakers as well

as SF6 circuit-breakers are suitable for

switching the magnetizing currents of

unloaded transformers with overvoltages

lower than 3.0 pu. In special cases, for

instance when vacuum circuit-breakers

are used for switching dry-type

transformers in industrial installations,

the use of surge arresters is to be

recommended.

34 ABB Review 4/2000

Motors

When choosing circuit-breakers for

motor-switching duty, attention should

be paid to the problems of overvoltages

during operation. The target limit for

overvoltages of less than 2.5 pu is

obtainable with both technologies.Where

vacuum circuit-breakers are used for

switching small motors (starting currents

less than 600 A), measures may be

necessary to limit overvoltages due to

multiple re-ignitions; however, the

probability of this phenomenon arising is

low.

Capacitor banks

Both technologies are suitable for

restrike-free switching of capacitor

banks. When capacitors must be

switched back-to-back, reactors may be

necessary to limit the inrush currents.

The synchronous control of circuit-

breakers is an effective solution to this

problem. SF6 is specifically recom-

mended for applications with rated

voltages higher than 27 kV.

Arc furnaces

Arc furnace switching is often

characterized by frequent operation at

high current values and short intervals.

Vacuum circuit-breakers are particularly

suited to these service conditions.

Shunt reactors

SF6 circuit-breakers are suitable for

switching with overvoltages generally

lower than 2.5 pu. Where vacuum circuit-

breakers are employed, it may be neces-

sary under certain circumstances to take

additional measures to limit overvoltages.

Railway traction

In principle, both interrupting technologies

are well suited for this duty; however, in

the case of low-frequency applications

(eg, 16.67 Hz), vacuum circuit-breakers

are to be recommended.

Matching the circuit-breaker

to the task

Thirty years of experience in developing,

manufacturing and marketing both SF6

and vacuum medium-voltage circuit-

breakers worldwide has yielded up

ample evidence that neither of the two

technologies is generally better than the

other, and especially that they are

complementary from the application

point of view. Economical factors, user

preferences, national ‘traditions’,

competence and special switching

requirements are the decision-drivers

that favor one or the other technology.

Typical of such special applications is

the switching of dry-type transformers,

small-size motors, capacitors, arc

furnaces, shunt reactors and railway

traction systems. The need for ‘frequent

switching’ or ‘soft switching’ can be an

additional element influencing the

choice. In such cases, a comprehensive

study of the planned installation may be

needed to find the best answer. ABB has

the know-how and experience necessary

to provide unbiased advice and

assistance to users in choosing the

circuit-breaker most suitable for any

particular application.

References[l] D. Braun, W. Heilmann, A. Plessl: Application criteria for SF6 and vacuum circuit-breakers. ABB Review 4/89, 25–33.

[2] A. Plessl, L. Niemeyer, F. Perdoncin: Research for medium-voltage SF6 circuit-breakers. ABB Review 2/89, 3–10.

[3] IEC 60056 – High-voltage alternating-current circuit-breakers, Amendment 3, 1996-09.

[4] R. Borlotti, A. Giacomucci: A simplified LCA of SF6 medium-voltage circuit-breaker. 7th SETAC Symposium, Brussels, 1999.

[5] R. Tinggren, Y. Hu, L. Tang, H. Mathews, R. Tyner: Power factor controller – an integrated power quality device. 1999 IEEE Transmission and

Distribution Conference, New Orleans.

[6] C. Cereda, C. Gemme, C, Reuber: Synchronous medium-voltage circuit-breaker: ABB solution based on magnetic drive and electronic control.

15th International Conference and Exhibition on Electrical Power Distribution Engineering, CIRED, Nice, 1999.

Authors

Guenter Leonhardt ABB Calor Emag Oberhausenerstraße, 3340472 Ratingen, Germanyguenter.leonhardt@de.abb.com

Mauro MarchiABB Trasmissione e DistribuzioneVia Friuli 4, 24044 Dalmine (BG), Italymauro.marchi@it.abb.com

Giandomenico RivettiABB Power DistributionVia Friuli 4, 24044 Dalmine (BG), Italygiandomenico.rivetti@it.abb.com

Switchgear